A good leather chair has a particular kind of give to it, firm under you at first and then softer the longer you sit, and the leather itself warms up to the temperature of the room within a few minutes so it never feels cold or slick the way a cheap vinyl copy does. Switch on a lamp next to one, settle in with a book, and you notice how the leather creaks just slightly when you shift, a sound that gets better with age instead of worse.
In an ordinary room, a leather chair sits best as the one solid, grounded thing among softer stuff. Put it near a fabric sofa and a couple of linen cushions and it reads as the anchor rather than the odd one out, the material contrast is doing something good instead of clashing. It wants space around it to breathe, so in a smaller room I'd rather have one leather chair and nothing else competing for attention than try to force a matching pair in, which almost always crowds a space that could have handled one confidently. A single chair in the corner by a window, with just enough room to actually open a book without knocking a side table, does more for a room than a cramped pair ever will.
If you've got none and you're starting from scratch, go for a simple club or armchair shape in a mid brown or tan rather than anything with deep tufting or a dramatic recline mechanism, because the plain shape works with almost any other furniture you already own and it won't date the way a trend shape will. The tufted ones and the big reclining ones are great once you know you love the material, but they're a harder first purchase because they commit the whole room to a particular look.
What separates a chair that lasts from one that disappoints is almost entirely in what you can't see from across the room. Full grain or top grain leather over a hardwood frame, with joints that are glued and screwed rather than just stapled, will hold its shape and its seams for decades, while bonded leather over a particleboard frame starts peeling and sagging within a couple of years no matter how good it looks on day one. Run a hand along the seams when you can, they should lie flat with no fraying thread, and press down on the arm, if it flexes or creaks in a hollow way rather than a solid one, that's the frame telling you everything.
The real pleasure of a well made one is small and easy to miss, the way the leather develops a slightly different sheen exactly where your arm rests, a mark that's entirely yours and gets better the more you use the chair, not worse.